1. FIELD OF THE INVENTION
The present invention relates to a piezoelectric ceramic composition, and in particular to a piezoelectric ceramic composition suitable for use in a ceramic oscillator, ceramic discriminator, ceramic filter or surface wave elastic element.
2. BACKGROUND ART
The parts used in conventional electronic equipment and devices are lead-type resin molded products. Soldering is performed by inserting the lead wires of the parts into a printed circuit board and immersing in a molten solder bath the surface with no parts mounted on it. Thus, the mounted parts are not exposed to temperatures of 200.degree. C. or higher.
With the progress made in recent years in the miniaturization of electronic equipment and devices, the use of surface packaging and mounting is increasing. To solder parts in surface packaging, chip parts with no lead wires on them are provisionally mounted on a printed circuit board and the entire printed circuit board is heated in a reflow furnace. As a result, the chip parts mounted on the printed circuit board are exposed during soldering to temperatures in the reflow furnace of around 250.degree. C.
The materials used in conventional type piezoelectric elements include: a piezoelectric ceramic composition in which a piezoelectric property is improved by adding Cr.sub.2 O.sub.3, MnO.sub.2, Fe.sub.2 O.sub.3, Bi.sub.2 O.sub.3, etc. to a two-component ceramic composition of PbTiO.sub.3 --PbZrO.sub.3 to make a three-component composition; a piezoelectric ceramic composition in which a piezoelectric property is improved by adding Pb(Co1/3Nb.sub.2/3)O.sub.3, Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3, Pb(Mn.sub.1/3 Nb.sub.2/3)O.sub.3, Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3, etc. to a two-component ceramic composition of PbTiO.sub.3 --PbZrO.sub.3 to make a three-component composition; or a three-component ceramic composition of PbTiO.sub.3 --PbZrO.sub.3 --Pb(Yb1/2Nb1/2)O.sub.3 as disclosed in Japanese laid-open publication No. 3-36773.
However, conventional piezoelectric elements having lead wires are usually not exposed to temperatures of 200.degree. C. or higher, and the ceramic compositions used in these piezoelectric elements are not designed to be exposed to temperatures of 200.degree. C. or higher.
For this reason, when these conventional piezoelectric elements are mounted on electronic devices and are exposed for soldering to temperatures around 250.degree. C. in the reflow furnace, the mechanical quality factor (Qm) and electromechanical coupling factor (kp) decrease. It is difficult to obtain piezoelectric element chip parts with reliable heat-resistance properties.
Also, polarization occurs in conventional piezoelectric ceramic materials when they are exposed to temperatures of 200.degree. C. even when the material has a Curie point of 300.degree. C. or more. Further, piezoelectric properties, such as electromechanical coupling factor, decrease. In a ceramic oscillator using conventional piezoelectric material, the resonance frequency deviates extensively from the initial frequency over time.